Adaptation: A new concept in radiation biology and oncology and its clinical relevance

A new concept of adaptation and cross-resistance in radiation biology and oncology is introduced. It explains the apparent difference in the radiosensitivity of cells of the same type (both normal and tumor) that are located in different parts of the body. Oxygen tension, which varies considerably from one part of the human body to another, ranges from 20 mmHg to 159 mmHg (2.7-21%). When cells are adapted to live in areas with high physiologic oxygen tension (high normoxic) such as the epidermis and upper and lower airways, they develop better antioxidant mechanisms, which include free radical scavangers and DNA repair, than cells that live in lower physiologic oxygen tension (low normoxic). Because the mechanisms of cell-damaging effects of ionizing radiation are similar to those of oxygen, the cell that is resistant to oxygen toxicity also becomes resistant to ionizing radiation (and certain cancer chemotherapeutic agents). Based on this concept, it could be expected that some human tumors such as primary cutaneous malignant melanoma and lung cancers contain cells that are relatively resistant to ionizing radiation as a consequence of living in a high normoxic condition and the treatment of these tumors may require different strategies.